I picked up one of those cheap canvas garages from Costco, to keep a few things out of the weather. It's 10' wide, 20' long, and maybe 11' high at the peak.

Our area is a bit prone to windstorms. I'm in the woods, so it's largely protected from the wind, but I still want to make sure it stays put.

It comes with tent stakes, but the ground is too soft for that to be reliable.

I thought about trying to attach it to concrete pier blocks, since they are heavy, cheap, and aren't permanent if we want to move the garage later. I'd put one under each leg, and fasten the leg to the block.

The blocks are formed with a hole in the top, about 5/8" diameter. The garage comes with some concrete anchors, presumably for use if you put it on a concrete driveway - you'd drill holes and secure it there. The anchors are way too small for these holes, though. Even the biggest anchors at the local hardware store seem too small.

I could try to buy larger anchors online.

I could epoxy in some inverted screws.

I could try to find some old railroad ties and screw in to those, instead. I'm not sure where to find those around here, however.

I could pour small concrete footings, although then I'm locked in to an exact setup.

" I'm in the woods, so it's largely protected from the wind, but I still want to make sure it stays put."

Tie it to the trees. If you're willing to dig a little find a root, rope under the root, and tie the tarp to that.

For a no-cement job try something like a 'snow anchor'. Tie your tarp down to a metal stake or a brick then bury that. The thing you tie to doesn't need to be heavy. The soil holds it down. Steel cable would be better than rope for this.

To use these anchors, first drive them into the hole (they should be relatively snug to begin with). Then as you tighten the nut around the object you want to secure, the wedge at the bottom expands and holds the bold in place within the concrete.

There are different styles of footings; for what you're dealing with, you don't need it to take the weight of the structure, you need it to resist lifting; for that, you can get away with fairly small holes, as you're actually more interested in the friction of the soil against the footing to anchor it down.

In my case, I have clay within 9 to 12", so for my greenhouse, I used a auger drillbit meant for planting flower bulbs (maybe 4" across, 24" long), to dig a few holes, then filled 'em with concrete and set the anchors that came with the greenhouse kit in each piling.

If I need to move the greenhouse, the pilings are small enough to dig up and move, and thin enough that I could even break 'em up with a sledgehammer and just remove the top so it won't interfere with a lawnmower.

We use these tents at shows and fairs. We always anchor them with screw in dog spikes just inside the poles, and tied straight down. I was in one during a storm with 60 mph gusts, and the tent only moved over one foot.